Carbon dioxide-rich atmospheres can form oxygen, confusing the search for life.

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Over the past 40 years, evidence has turned up on Mars pointing to the presence of oxygen. This suggested that some oxygen must have been created in the early Earth’s atmosphere as well, due to the similar compositions of the two atmospheres. Before this new idea, it was widely understood that oxygen in the Earth’s atmosphere originated in an event called the “Great Oxidation Event,” which occurred about 2.4 billion years ago as the first plants appeared and converted carbon dioxide to oxygen.

But a new experiment has confirmed that there is a mechanism for creating oxygen that doesn't require the presence of life. The results have implications not only for understanding the evolution of Earth’s atmosphere, but also for the study of exoplanetary atmospheres.

The experiment

The team used a vacuum ultraviolet (VUV) laser to break carbon dioxide apart, leaving free carbon and oxygen. Vacuum ultraviolet has a short wavelength (a range of 200-10 nanometers) that puts it at the far end of the ultraviolet portion of the electromagnetic spectrum. Today, VUV is absorbed by the oxygen in the atmosphere (hence its name). But in the early atmosphere, VUV from the Sun could have been producing oxygen out of then-abundant carbon dioxide.

By using the laser to excite the carbon dioxide, the team was essentially reproducing the natural VUV light from the Sun that would have been bombarding Earth’s early, CO2-rich atmosphere. Since the molecules were successfully broken apart, it means that the same thing should be happening in nature.

Once the carbon dioxide (CO2) was broken up into carbon (C) and oxygen (O2), the team used a unique ion imaging apparatus, developed at the University of California, Davis, to track the resulting particles, confirming that the CO2 remained separated. “Our study has provided unambiguous experimental evidence for the formation of C + O2 photoproducts,” the authors write in the paper.

Implications

The presence of oxygen on Mars was formerly explained by appealing to a complex process involving two steps. “Previously, people believed that the abiotic (no green plants involved) source of molecular oxygen is by CO2 + solar light — > CO + O, then O + O + M — > O2 + M (where M represents a third body carrying off the energy released in forming the oxygen bond),” Zhou Lu, of UC Davis, the paper’s lead author, told his university’s press office.

The new experiment provides a simpler, one-step mechanism, one that definitely could have been happening given the presence of VUV light from the Sun.

“Our results indicate that O2 can be formed by carbon dioxide dissociation in a one step process. The same process can be applied in other carbon dioxide dominated atmospheres such as Mars and Venus,” explains Lu.

The results of this experiment should be taken into account in future models of other planetary atmospheres that are rich in CO2, the paper’s authors argue. Theoretical understandings of exoplanetary atmospheres are important, as more and more exoplanets are discovered and as their atmospheres are analyzed. Some people have even argued that oxygen in the atmosphere of an exoplanet could be interpreted as a potential indication of life on its surface.

The reaction could even be happening now, and not just on Mars. In Earth’s upper atmosphere, there is CO2 present but little oxygen to absorb the VUV light. The Great Oxidation Event is still significant, but it's clear that oxygen could have significantly predated it.